1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method and an apparatus of relaying data in a wireless communication system based on Time Division Duplex (TDD).
2. Discussion of the Related Art
Methods for wireless communication systems may be chiefly classified into a TDD method and a Frequency Division Duplex (FDD) method. In accordance with the FDD method, uplink transmission and downlink transmission are carried out with them occupying different frequency bands. In accordance with the TDD method, uplink transmission and downlink transmission are carried out with them occupying the same frequency band at different times. The channel response of the TDD method is substantially reciprocal. It means that a downlink channel response and an uplink channel response are almost identical with each other in a certain frequency domain. Accordingly, there is an advantage in that in a wireless communication system based on TDD, the downlink channel response can be obtained from the uplink channel response.
In the TDD method, downlink transmission by a base station (BS) and uplink transmission by a mobile station (MS) cannot be performed at the same time because all the frequency bands are time-divided for uplink transmission and downlink transmission. In a TDD system in which uplink transmission and downlink transmission are classified on a subframe basis, uplink transmission and downlink transmission are carried out at different subframes.
Recently, wireless communication systems including a relay station (RS) are being developed. The relay station functions to expand the cell coverage and improve transmission performance. There is an advantage in that the cell coverage can be expanded because a base station provides service to a mobile station placed at the coverage edge of a base station via the relay station. Further, the transmission capacity can be increased because the relay station improves transmission reliability of a signal between the base station and the mobile station. In the case where a mobile station is placed within the coverage of a base station, but placed in a shadow region, a relay station may be used.
The relay station is difficult to send data to a mobile station while receiving data from a base station. Further, the relay station is difficult to send data to a base station while receiving data from a mobile station. This is because a signal outputted from the transmission stage of the relay station may interfere with a signal that should be received by the relay station under the influence of self-interference, distorting the signal.
In the wireless communication system based on TDD, uplink transmission and downlink transmission cannot be performed at the same time. Accordingly, the wireless communication system based on TDD can have lower resource efficiency in the time domain than a wireless communication system based on FDD. Furthermore, if a relay station is used in the wireless communication system based on TDD, radio resources must be allocated to the relay station. As described above, the relay station cannot perform the reception of data from a base station and the transmission of data to a mobile station (or the transmission of data to the base station and the reception of data to the mobile station) at the same time. Thus, the efficiency of resource allocation in the TDD system can be further lowered.
Accordingly, there is a need for a method of a relay station efficiently relaying data in a wireless communication system based on TDD.